Organic photovoltaics (OPVs) have seen significant progress over the last few years. A key milestone in this field has been the development of bulk heterojunction (BHJ) blends as the photoactive layer. In a BHJ solar cell, an electron donor (hole-transporting, p-type) semiconductor material and an electron acceptor (electron-transporting, n-type) semiconductor material typically are blended in solution. The mixture then is cast via solution-phase techniques onto one of the electrodes (e.g., a high work function indium tin oxide functioning as the transparent anode), with the donor and acceptor phases separating during the solvent drying process to form the BHJ photoactive layer, which has the morphology of a bicontinuous interpenetrating network. A low work function metal such as Al or Ca usually is deposited as the top layer which functions as the cathode. FIG. 1 illustrates a representative structure of an OPV cell. Due to dramatically improved donor-acceptor interfacial area, OPV cells based upon BHJ blends usually have much better performance than planar bilayer structures.
While designing novel materials is critical to continued improvements in OPV device performance, recent research has been focused mainly on the development of new conjugated polymers as donor materials, with soluble molecular fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (C60PCBM or PCBM) and [6,6]-phenyl-C71-butyric acid methyl ester (C70PCBM) remaining the dominant acceptors. Although fullerene derivatives show excellent charge separation behavior with a wide variety of donor materials and good electron transport, their absorption in the visible and NIR region is limited. In addition, their lowest unoccupied molecular orbital (LUMO) energy level, the governing property for the open circuit voltage (Voc) of OPVs, is fixed and cannot be easily adjusted. Therefore, the two major loss mechanisms in today's OPVs are the low photocurrent (Jsc) due to insufficient photon absorption and the low Voc compared to the band gap of the absorbers due to non-optimum LUMO-LUMO offset of donor and acceptor materials. In addition, in terms of processing, the use of an acceptor polymer (instead of a molecular acceptor like a fullerene derivative) with a donor polymer will allow more uniform films to form over large areas, hence facilitating large-scale production of OPV modules.
Efforts to replace fullerene derivatives with other organic acceptor materials have not been very successful to date. Particularly, the approach of using n-type polymers as acceptors in OPVs has not yielded high power conversion efficiencies (PCEs) even though a range of materials with good electron transporting properties and good absorption in the visible and NIR are available. Particularly, one of the observations is that electron-transporting or n-type polymeric semiconductors that show high performance in thin film transistor (TFT) applications do not excel necessarily as OPV acceptors. See Anthony et al., “N-Type Organic Semiconductors in Organic Electronics,” Adv. Mater., vol. 22, no. 34, pp. 3876-3892 (2010). To date, no PCE over 3% has been reported for all-polymer, fullerene-free OPVs.
For example, different groups have investigated OPVs based upon the combination of poly(3-hexylthiophene), P3HT, as the donor material and poly([N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′- bithiophene)), P(NDI2OD-T2), as the acceptor material. First studies yielded very low PCEs of ˜0.2%. See Moore et al., “Polymer Blend Solar Cells Based on a High-Mobility Naphthalenediimide-Based Polymer Acceptor: Device Physics, Photophysics and Morphology,” Adv. Energy Mater., vol. 1, no. 2, pp. 230-240. Significant improvements in PCE to 0.6% were achieved through improved processing solvent, and then to 1.4% by controlling the aggregation of P(NDI2OD-T2) through solvent mixtures and hot solvent processing. See Fabiano et al., “Role of Photoactive Layer Morphology in High Fill Factor All-Polymer Bulk Heterojunction Solar Cells,” J. Mater. Chem., vol. 21, no. 16, pp. 5891-5896; and Schubert et al., “Influence of Aggregation on the Performance of All-Polymer Solar Cells Containing Low-Bandgap Naphthalenediimide Copolymers,” Adv. Energy. Mater., vol. 2, no. 3, pp. 369-380.
Accordingly, the art desires new polymeric blends that can enable high-efficiency all-polymer OPV devices.